Paper and paperboard products are frequently coated in order to form a barrier against gases such as moisture; liquids such as water, oils or other solvents; and solids such as ice and greases. Should the paper be used as a container, it is advantageous to have a coating which acts as a barrier to both internally or externally sourced gases, liquids or solids such as those mentioned above.
Another benefit of coating paper is to render the paper relatively nonabrasive so as to prevent a container made from such paper from damaging, or being damaged by, the container contents or adjacent surfaces This can occur for instance when a paper container and its contents are exposed to the vibration attendant to long distance travel such as by truck or train.
Another reason for coating paper is to increase the structural integrity of the paper by rendering it more rigid Thus, a container made of coated paper has more strength both under normal conditions and in more hostile environments, such as moisture-containing environments and/or corrosive environments.
Yet another reason for coating paper is that it allows a manufacturer to apply a color and/or gloss to the paper to make it more appealing to the customer or user of the paper product or its contents.
There are many types of widely used commercial coatings examples of which include solvent-borne coatings, water-borne coatings, powder coatings, radiation curable coatings and hot melt coatings. The present invention relates to hot melt wax coatings which enjoy some advantages over other types of coatings including low cost, ease of application and not requiring the application and subsequent evaporation of greater amounts of solvent or water on and from the paper substrate.
In spite of the substantial advantages afforded by hot melt coated papers, such materials suffer from one major disadvantage in that they cannot be easily recycled and must, therefore, be disposed of as waste (such as by burning) or used as landfill. In recent years, increased environmental awareness has made these alternatives increasingly less desirable, if not illegal, and impracticable. Specifically, the hot melt coating cannot be readily separated from the paper substrate, and the hot melt coating does not break down in aqueous environment of a pulping machine, such as those used in the recycling of paper products. As a result, the hot melt coating contaminates and clogs the pulping and paper-making machinery. The hot melt coated papers of the prior art have not been recyclable and have thus rendered a potentially valuable pulp resource a serious waste disposal problem.
Hot melt coating materials are, of course, well known and can be obtained in the form of molten liquids or thermoplastic slabs, flakes or pellets. Typical examples of such materials include natural and/or synthetic plastics and waxes as set forth, for instance, in U.S. Pat. No. 3,058,871 to Davis, U.S. Pat. No. 2,859,110 to Sanford and U.S. Pat. No. 3,021,252 to Hill. Such thermoplastic materials include, for instance, paraffin, microcrystalline wax and other waxes, which may be used with additives such as hydrocarbon resins and synthetic polymers.
As stressed in the aforementioned U.S. Pat. No. 3,058,871 to Davis, numerous prior attempts have been made to recover pulp from hot melt coated waste paper. However, it has been generally found that the waxes and plastics present on and impregnated in such paper are difficult to separate effectively without seriously contaminating the pulp and the paper-making machinery. In U.S. Pat. No. 3,058,871 to Davis and U.S. Pat. No. 2,703,754 to Myers, for example, attempts were made to separate the hot melt coating from the pulp by solvent extraction of the coating. In U.S. Pat. No. 3,055,791 to Elias, solid absorbents were used in an attempt to recover pulp, whereas in U.S. Pat. No. 3,021,252 to Hill and U.S. Pat. No. 2,859,110 to Sanford, the coating was mechanically separated from the fiber. In U.S. Pat. No. 2,703,754 to Myers, the separation of pulp and coating material was made by a combination of emulsification and solvent extraction.
The most frequently suggested procedure for recycling hot melt coated paper involves suspending the coating particles in a hot aqueous system as suggested in U.S. Pat. 3,822,178 to Von Koeppen et al, U.S. Pat. No. 2,614,922 to Hope, U.S. Pat. No. 2,859,110 to Sanford and U.S. Pat. 2,959,513 to Savage.
All of the above-discussed patents are hereby incorporated herein by reference Of the above, perhaps the most significant process is described in the Von Koeppen et al patent. It is pointed out in this prior art patent that if paper cannot be recycled, serious disposal problems result. However, as noted above, paper mills have not been able to recycle waxed paper stock in the past because of process and equipment difficulties. In order to enable waxed paper stock to be used in Fourdrinier, cylinder and wet-lap processes, Von Koeppen et al suggests charging a pulper or beater with hot melt coated or impregnated paper and combining this charge with water and certain nonionic emulsifiers at a temperature high enough to melt the hot melt coating. Although, as a result of emulsification, substantial quantities of wax, or other paper coating material, are extracted in the emulsified phase, an unsatisfactorily high amount of the coating material remains unemulsified in the system.
Thus, whereas emulsification of paper coatings is perhaps the most promising coated paper recycling process, it has not been totally satisfactory and is not in widespread commercial usage. In fact, as stated in U.S. Pat. 3,055,791 to Elias, removal of organic coatings by emulsification has been attempted without appreciable success.
Although not to be limited by theory, applicant suggests that the lack of success experienced by these prior art coatings and emulsification processes is due, in large part, to the lack of immediate intimate mixing of the emulsifier with the coating and thus the lack of complete emulsification. The coatings tend to float in a pulper and physically segregate themselves from emulsifiers charged therein, making the coatings difficult to emulsify.
Accordingly, it is a primary object of the present invention to provide a latent dispersant, surfactant or emulsifier (or a chemically modifiable precursor thereof) incorporated into the hot melt paper coating itself to overcome the shortcomings of the prior art discussed above.
It is another object of the invention to provide an emulsification process wherein adequate emulsification of hot melt coated paper is achieved to permit such coated paper to be commercially recycled.
A further object of the invention is to create a pulping mixture which does not require a physical separation between a hot melt coating and the paper in a pulper during a recycling process.
Yet another object of the invention is to provide a paper with a hot melt coating which may be readily and adequately emulsified during recycling in a conventional repulping process without deleteriously affecting the process equipment or resulting paper. The type of pulper which may be used in repulping the coated paper of the present invention include any of those pulpers known in the art such as a Hydropulper, Hollander and the like. The slurry can then be transferred to any type of processing equipment, whether Fourdrinier, cylinder, or other equipment.
Another object of the present invention is to provide a hot melt coating which can be readily emulsified or dispersed during a repulping process.
Additional objects, advantages, and other novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention. The objects and advantages of the present invention may be realized and attained by means of the compositions and methods particularly pointed out in the appended claims, including the functional equivalents thereof